1. Field of the Invention
The present invention concerns a photoelectric conversion apparatus and a driving method thereof. More particularly, the invention relates to a photoelectric conversion apparatus and a driving method of the photoelectric conversion apparatus suitably used in imaging apparatus utilizing visible rays or radiations typified by X-rays, for example, in one-dimensional or two-dimensional imaging apparatus such as still cameras or radiation imaging apparatus.
2. Related Background Art
Most conventional photographs were silver-salt photographs taken using an optical camera and silver-salt film. Although the semiconductor technology is progressing to develop imaging apparatus that can pick up moving pictures, such as a video camcorder using a solid state image sensing device comprising Si single-crystal sensors typified by CCD sensors or MOS sensors, these images are inferior in the number of pixels and in S/N ratios to silver-salt photographs. It was thus normal practice to use the silver-salt photographs for taking still images.
On the other hand, a desire exists in recent years for electronic imaging apparatus that can output digital signals of images comparable to the silver-salt photographic images with increase in demands for image processing by computer, storage by electronic file system, and transmission of image by electronic mail system. This is also the case in the fields of examination and medical care as well as the ordinary photography.
For example, the X-ray photography is generally known as photography using the silver-salt photographic technology in the medical field. This is used in such a way that X-rays emitted from an X-ray source irradiate the affected part of a human body and that information of transmission thereof is used to judge presence or absence of fracture or tumor, for example, and has been widely used for medical diagnosis for a long time. Normally, X-rays transmitted by the affected part are made incident once into a fluorescent body to be converted thereby to visible light and the silver-salt film is exposed to the visible light. However, while silver-salt film has advantages such as high sensitivity and high resolution, it also has disadvantages, such as the long time necessary for development, the labor required for preservation and management, the inability of sending an image developed thereon quickly to a remote location, etc. Therefore, a desire exists for electronic X-ray imaging apparatus that can output digital signals of images comparable to the silver-salt photographic images, as discussed previously. Of course, this is the case not only in the medical field, but also in non-destructive examination of sample (detected object) such as a structural body.
Responding to this desire, imaging apparatus has been developed that uses a large-scale sensor in which image pickup elements using photoelectric conversion elements of hydrogenated amorphous silicon (hereinafter referred to as a-Si) are arrayed two-dimensionally. The imaging apparatus of this type is constructed, for example, in such a manner that a metal layer, an a-Si layer, and the like are deposited on an insulating substrate with each side ranging from 30 to 50 cm approximately, using a sputter system, a chemical vapor deposition system (CVD system), and the like, semiconductor diodes, for example, of approximately 2000.times.2000 are formed, an electric field of a backward bias is applied to the diodes, and thin-film transistors (hereinafter referred to as TFT) also fabricated at the same time are used to detect respective charges of these individual diodes flowing in the backward direction. It is widely known that when the electric field in the backward direction is applied to a diode of semiconductor, photocurrent flows according to a quantity of light incident to the semiconductor layer, and this is utilized. However, an electric current so called dark current flows even in the state of no incident light, which causes shot noise. This shot noise is a factor to degrade detection performance of the total system, i.e., to lower the sensitivity, i.e., the S/N ratio. This could negatively affect judgment of medical diagnosis or examination. A misfocus or inaccurate evaluation of a defective part due to this noise can be a problem. It is thus important to reduce this dark current as much as possible.
It is also known that continuous application of the bias to the semiconductor diodes or other photoelectric conversion elements could cause the electric current flowing therein to increase imperfections in semiconductor, thereby gradually degrading their performance. This will appear as phenomena of increase of the dark current, reduction of the electric current due to light, i.e., reduction of photocurrent, and so on. In addition to the increase of imperfections, the continuous application of electric field could also cause a shift of threshold of TFT and corrosion of metal used for wiring because of movement of ions and electrolysis, thereby lowering reliability of the total system. Low reliability would raise a problem in bringing medical care equipment or examination equipment to the commercial stage. For example, failure must not occur during diagnosis, treatment, or examination in an emergency. The above description concerned the sensitivity and reliability with the example of semiconductor diodes, but the problems are common to the photoelectric conversion elements of various types without being limited to the diodes.
FIG. 1 is a schematic block diagram to show an example of the X-ray imaging apparatus. In FIG. 1, reference numeral 1 designates a sensor section in which many photoelectric conversion elements and TFTs are formed on an insulating substrate and in which an IC and the other circuits for controlling these are mounted. Roughly speaking, the sensor section has three terminal parts, a bias applying terminal (Bias) for applying an electric field to the photoelectric conversion elements, a start terminal (START) for supplying start signals of reading and initialization, and an output terminal (OUT) for outputting outputs from the respective photoelectric conversion elements arrayed two-dimensionally in the form of serial signals. Numeral 2 denotes an X-ray source, which emits pulsed X-rays under control of a control circuit 5. The X-rays pass through an examined part of a detected object such as the affected part of patient and the passing X-rays including information thereof travel toward the sensor section 1. A fluorescent body exists between the sensor section 1 and the detected object, though not illustrated, so that the passing X-rays are converted to visible light. The visible light after conversion is incident to the photoelectric conversion elements in the sensor section. Numeral 3 is a power supply for applying the electric field to the photoelectric conversion elements, which is controlled by a control switch (SW), or by the control circuit 5.
The above-stated apparatus, however, had improvable points as described below.
FIGS. 2A to 2D show an example of the operation of the X-ray imaging apparatus shown in FIG. 1. FIG. 2A to FIG. 2D are schematic timing charts each to show the operation in the imaging apparatus. FIG. 2A shows the operation of the imaging apparatus. FIG. 2B shows the X-ray emission timing of the X-ray source 2. FIG. 2C shows the timing of the bias applied to the photoelectric conversion elements. FIG. 2D shows the electric current flowing in the photoelectric conversion elements. FIG. 3 is a flowchart to show the flow of the operation.
In FIG. 2A before the arrow indicated by (SW ON) no bias is applied to the photoelectric conversion elements as shown in FIG. 2C (Bias OFF). Here, detection of &lt;SW ON?&gt; 301 is carried out as shown in FIG. 3. If the control switch (SW) is flipped on then [Bias ON] 302 will be effected. This is also shown in FIG. 2C. At the same time as it, charges of the individual photoelectric conversion elements in the sensor section 1 are initialized as shown by Int. of FIG. 2A and [Initialize Sensors] 303 of FIG. 3. After completion of the initialization, the control circuit 5 controls the X-ray source 2 to emit X-rays. This causes the imaging apparatus to perform exposure (Exp. of FIG. 2B and [Exposure] 304 of FIG. 3). After this, charges including optical information, having flowed in the individual photoelectric conversion elements, are read by the operation of internal TFTs and IC, as shown by Read of FIG. 2A and [Read Sensors] 305 of FIG. 3. After that, the electric field to the photoelectric conversion elements is turned to 0 (OFF) as shown in FIG. 2C or by [Bias OFF] of FIG. 3. Then the apparatus stands by until the control switch is next flipped on.
In the above operation, however, the electric current of the photoelectric conversion element is large before and after exposure as shown in FIG. 2D. Semiconductors, especially amorphous semiconductors such as a-Si, have a great dark current immediately after application of bias, so that the electric current flows despite no light incidence for a while. This is the influence of shot noise discussed previously and indicates a possibility of failure in reproduction of good X-ray image. This could result in failing to give appropriate diagnosis or examination. The reason of this dark current explained is such that the change of the electric field in semiconductor makes a place in which the Fermi level in the forbidden band moves relatively, which moves electrons and holes at the trap near the center of the forbidden band to cause the dark current. This trap results from imperfections of semiconductor and discontinuity of crystal structure at the interface between semiconductor and insulator, and thus the increase of dark current occurs in the photoelectric conversion elements of any material or any structure. Also, charges of ions or the like move immediately after application of the electric field, and an unstable electric current flows before stabilization thereof, which is another cause.
FIGS. 4A to 4D show another example of the operation of the X-ray imaging apparatus. The block diagram of the whole apparatus is the same as that of FIG. 1 and omitted herein. In FIGS. 4A to 4D like operations and representations are denoted by the same symbols as in FIGS. 2A to 2D. Most of the operations are the same as those in FIGS. 2A to 2D described above, but a different point is that the electric field is continuously applied to the photoelectric conversion elements as shown in FIG. 4C. Namely, as also seen in FIG. 5, the Bias ON state is maintained without providing [Bias ON/OFF] in the sequential operation for exposure. This decreases the dark current as shown in FIG. 4D, when compared to the operation shown in FIGS. 2A to 2D, and this seems to achieve a good image. This operation, however, includes a hidden problem in fact and cannot be employed as a product. The reason is that this operation requires the electric field to be always applied continuously to the photoelectric conversion elements during a period of time as long as the apparatus is possibly used, for example, for clinic examination hours of hospital. For example, supposing the imaging operation is carried out 100 times per day and for three seconds per person with the operation of FIGS. 2A to 2D, the time for application of electric field to the photoelectric conversion elements is 300 seconds in total. In contrast, supposing the possibly used period such as the clinic examination hours is 8 hours, the operation of FIGS. 4A to 4D continues about 30,000 seconds, which is the operation condition approximately 100 times longer. This results in also applying the electric field to the photoelectric conversion elements during the other periods than the actually photographing periods (i.e., during the non-operative periods). This will also lower the reliability as discussed previously and does not suit practical use, also taking the maintenance fees and the like into consideration.